1. Field of the Invention
The present invention generally relates to high temperature thermocouple assemblies and particularly to immersible molten metal temperature measuring devices having a long life in molten metal temperature measurement applications and corrosion resistance to such immersion in molten metals such as aluminum alloys and steel.
2. Description of the Prior Art
In melting and casting metals such as aluminum alloys and in steel making the continuous measurement of the temperature of the molten metal is of great importance in controlling the quality of the product and the profitability to the metal manufacturer. Continuous temperature measurement is necessary to produce castings of zero defect and to enhance productivity in general. It is essential that a continuous reading of the metal bath temperature is kept within control limits during metal melting and casting so that this information may be accurately and continuously feed into control systems to allow decision making for optimum times to initiate the casting and pouring of the metal for various manufacturing processes.
To produce thermocouple which are suited for measuring the elevated temperatures found in molten metals, various devices are know such as ceramic protectors for thermocouple. These known devices suffered from an early failure of the thermocouple device upon actual use beyond short undesirable time period. This early failure resulted in a great loss for the metal producer because of the repeated interruption in production necessary to replace failed thermocouples resulting in a more costly product. Therefore, thermocouple assemblies were needed which provided durability, integrity and which were resistant to erosion, dissolution and rupture upon insertion in high temperature molten metals. The terms "high temperature" and "elevated temperature" as used herein refers to temperatures, generally within a range of approximately 538-1550.degree. C. Criteria considered in selecting thermocouple for use in molten metal requires longevity of the devise under consideration. It is of paramount importance to increase the usage time of the thermocouple in order to not interrupt production especially in industries using metal casting and tapping of metals from a furnace and especially in recycling hearth furnaces for aluminum recycling and purification. In industry, to obtain quality controlled product and measuring parameters which affect metal refining, prevention of segregation within the metal, continuous measuring of temperature in continuous casting, are of a paramount importance for consideration in recommending and use of thermocouple assemblies.
Known thermocouple assemblies use certain ceramics and cements to protect thermocouple leads which constitute the sensor device. Many references appeared in prior U.S. patents with emphasis in incorporating protective tubes made of materials which possess certain physical and chemical properties. These properties usually incorporate low thermal expansion, high melting point, stability (insoluble) in molten metal, rupture strength and low cost. Also, the relationship, dimensions of the parts included in constructing the thermocouple assemblies are of utmost importance in order to make a feasible and usable thermocouple.
Known materials for such applications are refractories, and glass covering protectors for same. Such materials are disclosed in three U.S. Pat. Nos. 4,060,095; 4,135,538 and 4,216,028 by Kruita. These U.S. patents describe configurations and assemblies for thermocouple protective tubes composed of multiple layers of ceramic paper formed into a tube protected by a silica glass cover. The above described thermocouple assemblies were usable for several hours in measuring molten metal temperature before they had to be replaced. Of course the process using these thermocouples had to be shut down to effect a replacement of the thermocouple. This replacement requirement slowed down the melting process adding to the cost of the ultimate product.
Other known high temperature thermocouple assemblies are described in U.S. Pat. No. 5,654,034. These devices use SiC,CVD formed on the end of the thermocouple assembly. Other high temperature devices are also known but their applications involved measuring air temperature in gas fired burners and obviously not molten metals.
The problem of thermocouple wear in molten metal also needed improvement for corrosion and wear resistance.
Thus what was needed was a thermocouple protection tube which will provide a thermocouple assembly for molten metal temperature measurement which is both shock and wear resistant, easy to use and assemble and has a longer life in the order of days rather than a few hours.